Portland cement is commonly used in cement and concrete mixtures for a variety of commercial and individual purposes such as for building structures, roads and highways, bridges, concrete pipe and in the formation of myriad precast construction components. Presently, many different types of Portland cement are produced in the United States with specifications for each being dictated by the intended end use.
The raw materials involved include limestone, clay, shale, iron containing materials and siliceous materials. These materials are normally obtained by drilling and blasting techiques with the raw materials then being transported from the quarry to crusher and screening operations.
The crushed and/or screened materials are then further processed to obtain "Portland cement clinker" which clinker consists essentially of hydraulic calcium silicates. Portland cement is formed by adding gypsum and then pulverizing the mixture by means of a ball mill or the like. It is then either packaged or stored. The end user then adds water, aggregate, or sand, etc. to the dry ground clinker-gypsum mixture to form the desired mortar or concrete (end product).
The raw materials obtained from the quarry are usually subjected to either a "dry" or "wet" process prior to calcination. For example, in the "dry process", the raw materials are dried and ground. The ground particles are then separated by size, with overly large particles being re-ground. The products are then blended and stored prior to calcination. In the "wet process", the raw materials are slurried and then ground in slurry state prior to calcination.
In the calcination step, the blended material, from either the "dry" or "wet" process, is fed to an elongated rotating kiln operating at temperatures up to about 2700.degree. F. The product exits the kiln at temperatures from about 1800.degree. F. to 2700.degree. F. and is referred to as cement clinker. The clinker is then cooled via air draft circulation or similar means, stored, then normally blended with gypsum and sent to a rotary grinding or finishing mill. The product is then stored prior to packaging and bulk transport by rail, barge or truck.
Although the problem of fugitive dust emissions may be present at various points through the Portland cement process, it is especially troublesome when the hot clinker (e.g., 150.degree. to 400.degree. F.) is transferred after the clinker cooler to a conveyor upstream from the finishing mixing mill step or as the cooling clinker is transferred to a storage pile.
In order to minimize the emission of fugitive dust emanating from the raw materials or from the clinker, numerous approaches have been tried. Typically, process transfer points may be hooded or covered. Attempts at wet dust suppression have been generally ineffective. Clinker may actually prematurely "set-up" or cake, adversely affecting the commercial value and performance of the final cement product.
Experiments have been undertaken with the use of foamed dust control treatments. Despite the superior dust control efficacy of foamed treatments, the foam results in an increase in entrained air in the resulting cement products. This factor has severely curtailed use of foams since the increased air entrapment causes undesirable variation from the air entrainment specifications for particular Portland cement grades. More importantly, undesirable increases in the air entrapped within the product may lead to product compressive strength reduction that could cause structural failure of the cement.
These and other problems in the art of cement clinker dust control are addressed by the present invention.